I can’t edit the text on a ebook template image
Welcome to the Unbounce community.
You can’t edit the text on the cover of the book because it is an image.
The text is not an element from the editor.
You have a few options here:
Modify the image through a software like Photoshop or Sketch that allows you to work with image files.
Find a .PSD file of a book/book cover that would allow you to edit it.
Hope this points you in the right direction.